Type inference worked in 4.2.3 but fails in 4.3.2

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
typescript
Domain
compilers

Research direction

Start with the linked TypeScript Playground repro and compare inference between versions 4.2.3 and 4.3.2, focusing on the null choice in the choices array and the promptChoice call. Done means the example no longer reports the tuple assignment error and choice is inferred as 'one'|'two'|'three'.

Written by the indexing model from the issue text.

Description

Bug Domain: check: Type Inference

Bug Report

🔎 Search Terms

type inference

🕗 Version & Regression Information

4.3.2

  • This is a crash
    [x] This changed between versions 4.2.3 and 4.3.2
  • This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
  • I was unable to test this on prior versions because _______
⏯ Playground Link

Playground link with relevant code

💻 Code

(strict null checks off)

interface PromptChoiceOptions<T extends string> {
	label: string;
	button: string;
	default?: T;
	choices: readonly ( T | [ string, T ] )[];
}

function promptChoice<T extends string>( options?: Partial<PromptChoiceOptions<T>> ) {
    return options?.choices?.[ 0 ] ?? null; // simplified
}

const choice = promptChoice( { label: 'example', button: 'select', choices: [
    [ 'null', null ], // problem seems to be related to this line
    [ 'One', 'one' ],
    [ 'Two', 'two' ],
    [ 'Three', 'three' ]
] } );
🙁 Actual behavior

Type [string, "two"] is not assignable to type [string, "one"]

🙂 Expected behavior

choice is inferred to be 'one'|'two'|'three'

In creating a minimal repro for this, I discovered that commenting out the null choice makes the code behave properly. Also, changing the type of choices to readonly ( T | [ string, T|null] )[]; with strict null checking makes the problem always occur.

Dominant language
Go
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Merged PRs (30d)
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